This Invention relates to an optical fiber transmission path in which the effects of dispersion are minimized or compensated for.
Among conventional single mode optical fibers, fibers having specific refractive index difference 0.19% and core diameter 9.4 .mu.m have been reported to achieve 0.2 dB/km low loss, close to the theoretical limit at wavelength 1.55 .mu.m (T. Miya et al., Electron. Lett., 15 (1979) 106). However, the wavelength at which the total dispersion of this silica-based (hereinafter SiO.sub.2 -based) optical fiber will become zero is 1.32 .mu.m. The total dispersion at the wavelength 1.55 .mu.m, which provides the lowest loss value, is reported to be -1.7 ps/.ANG./km (A. Sugimura et al., IEEE. J. of QE, 16 (1980) 215). The realization of an ideal transmission medium with dispersion at wavelength 1.55 .mu.m minimized is also discussed.
Material dispersion is inherent in the optical fiber material; therefore it is possible to control waveguide dispersion to shift the zero dispersion wavelength to a wavelength greater than 1.32 .mu.m. Proposed solutions include making the relative refractive index difference between the ore and the clad larger, and achieving zero total dispersion over a wide range of wavelength by providing a W type refractive index distribution See, e.g., Nishida et al. U.S. Pat. No. 3,997,241. Additional structures for such "dispersion flattening" are disclosed in Bhagavatula, U.S. Pat. No. 4,715,679.
However, in these cases, a special waveguide structure has to be formed and problems, such as increase in scattering may occur. These are difficult to solve with special core designs. Recently, an approach is reported in which the relative refractive index difference of core-clad is increased by using a pure SiO.sub.2 core and by doping fluorine in the clad. By using a pure SiO.sub.2 core, the scattering is at kept low in an attempt to achieve a low loss, at the 0.2 dB/km level, in fibers having zero dispersion at 1.55 .mu.m. According to recent reports (for example: Tanaka et al., National Conference Record 1987, Semiconductor Devices and Materials, The Inst. of Electronics Information & Communications Engineers (1987) p. 2-218; and, Tanaka et al., ibid., 430 (1987) p. 2-217), by providing a special waveguide structure, such as dual shape type or step added Gaussian type, etc., the zero dispersion can be shifted to 1.55 .mu.m and 0.2 dB/km transmission loss can be achieved.
It has been suggested that a fluoride glass-based fiber made from BGZA matrix glass, connected to a SiO.sub.2 -based fiber, be used in order to accomplish dispersion compensation (Mitachi et al. Japanese Patent Publication 60-121403; S. Mitachi, Society of Glass Technology Conference on Infrared Transmitting Materials, Peebles, June 12, 1986, Abstract D; and, S. Mitachi, IECE Japan National Conference Record, Nov. 1-4, 1987, Kumamoto, Technical Paper P. 2-223). However, the transmission path disclosed in these publications requires relatively long lengths of fluoride glass optical fiber, which are difficult to fabricate, for this application, since the zero material dispersion wavelength of fibers made from BGZA matrix compositions is only in the range of 1.67-1.7 .mu.m. The total dispersion at the preferred operating wavelength for minimum attenuation of the SiO.sub.2 -based fiber, 1.55 .mu.m, is approximately 1 ps/.ANG./km. The typical zero material dispersion wavelength of silica-based optical fibers is approximately 1.3 .mu.m whereas the preferred operating wavelength for minimum attenuation is approximately 1.55 .mu.m, where silica-based single mode fiber has total dispersion of -1.6 ps/.ANG./km. Therefore, a 1.6 times longer fiber made from BGZA matrix compositions is necessary to compensate for the dispersion in SiO.sub.2 -based fibers at the 1.55 .mu.m operating wavelength. For example, if SiO.sub.2 -based fibers of 10km length are used, BGZA fluoride fibers with 16km length are needed.